A data processing method, a communications device, and a communications system, where in a process of transmitting a packet service, a code block stream carrying the packet service is first obtained, then, rate adaptation is performed on the obtained code block stream, and finally, the rate-adapted code block stream is mapped to an optical channel payload unit (OPU) signal. Compared with a mapping manner in which a Generic Framing Procedure (GFP) is used, the data processing method, the communications device, and the communications system feature low processing complexity and/or high bandwidth utilization.